So how does our body digest our food?

It takes one to three days, but we should be aware of also not eating too much

YOLANDA VEIGA

The plate of lentil stew you have just eaten has begun a long journey through your digestive system. The 'tour', which lasts between one and three days, starts in your mouth and ends (if you'll excuse me for saying so) in the lavatory. Here is what happens at each stage.

Mouth

"The teeth called incisors cut the food, which is why they are shaped like knives, and the molars, which are like a pestle and mortar, grind it up. In addition to crushing the food up so it doesn't get stuck in the oesophagus, which is quite a common problem, especially when eating meat, the saliva contains substances which start to digest the food, such as amylase, a protein that breaks down carbohydrates," explains Enrique de Madaria, president of the Spanish Gastroenterology Association and a specialist in digestion at the Doctor Balmis general university hospital in Alicante.

The food begins by being crushed in your mouth and ends up as a very fine, liquid-like mass

Chewing is essential to prevent choking and to facilitate digestion. A lot of chewing, in fact. "Some people say chew every mouthful 30 times, but it depends on the texture of the food. The idea is to chew more times than you normally would, not just so the stomach has less work to do but because it is also good for dental health and it helps to create a feeling of fullness, which is important so we don't eat too much," says Eduard Baladía of the Spanish Academy of Nutrition and Dietetics.

Oesophagus

This is the simplest organ in the digestive system, a muscular tube that takes what we swallow and transports it from the throat to the stomach.

"At the end there is a sphincter that relaxes to let the food through and then closes again, because the inner lining of the oesophagus is not designed to accept acid from the stomach," says Dr de Madaria. If this 'gate' fails, the acid (pH2-4) rises and damages the mucous membrane, which is what causes heartburn.

Stomach

"The stomach is in the shape of a bag and has a volume of about one litre, although its walls can stretch to take up to four," says Dr de Madaria. To give an idea of when it is full: one dish of lentil stew, two glasses of water and a piece of fruit would be one litre.

"It's not a good idea to stretch the stomach too much, because when it is the normal size it lets us know when we are full. If it is expanded more often it gets used to it and will need more to fill it and therefore we won't feel full so quickly," says Teresa Arnandis, a doctor of biochemistry and biomedicine and an author, who is better known on social media as Lady Science.

As well as storing what we eat and drink, the stomach produces powerful contractions that move the food and help to digest it. "It secretes acid and that acid actives the protein pepsin, which digests the proteins and kills bacteria, fungi and micro-organisms that we ingest with the food," says Dr de Madaria.

What comes out of the stomach is chyme, "a semifluid mass with particles of food measuring less than five milimetres," which passes through the pylorus, the closure at the end of the stomach. "The food leaves the stomach after about two to four hours," says Teresa Arnandis.

Small intestine

"We have two glands which are very important in digestion: the liver and the pancreas. The liver secretes bile and the pancreas secretes pancreatic fluid and they mix with the chyme," she explains.

The bile contains salts which act like a detergent for the body. It dissolves fats from the meal, while the pancreatic fluid has digestive proteins or enzymes which, on one hand, digest the fat that has been solubilised by the bile and, on the other, the rest of the components such as carbohydrates, and proteins.

In this phase the chyme becomes chyle, an even finer mass, which then begins its journey through the small intestine (which actually measures between six and nine metres).

"It contains intestinal villi, which are hairlike folds in the mucous membrane to increase the amount of surface area in contact with the content of the intestine. The cells in those tiny hairs contain enzymes that digest the food (for example, lactase digests lactose, so anyone who is lactose-intolerant does not have this enzyme on the ends of the hairs). At the same time, these villi are absorbing nutrients like sugars and aminoacids, so that all that remains is what can't be absorbed: vegetable fibres or waste components which go through to the large intestine," says Dr Arnandis.

Large intestine

"The large intestine is populated by our microbiota, which digest some components of the food which can still be of some use. In this phase, water is extracted from these remains of food, so it becomes less liquid and is fermented by the bacteria. This is the process that forms the faeces, which accumulate in the final part of the intestine (the rectum) and reach the anus, which is a sphincter that relaxes in order to release them," says Dr de Madaria.

This is also the point at which gases can form. "When the bacteria in the large intestine digest these remains of the food which can't be absorbed, they ferment them and that is what produces the gases," he explains.